Using neutron scattering to study the structure and flexibility of hexadecyltrimethylammonium dichlorobenzoate and related micelles

Certain ionic surfactants can self-assemble in aqueous solutions into giant flexible wormlike micelles that behave similarly to semi-flexible polymers. Small angle neutron scattering (SANS) has been used to study the sizes and flexibilities of these micelles. By allowing the probing of shorter length scales than light scattering, SANS is clearly preferable for determining micellar persistence lengths directly. SANS curves were fit to a series of micellar models to get information on persistence lengths, contour lengths and cross-sectional radii. The impact of different tail lengths, head groups, counterions, and varying ionic strengths were studied. Counterions have major effects on micellar size and flexibility. For nonpenetrating counterions, such as chloride and bromide, the micellar size and flexibility increase as the concentrations of the counterion increase (by adding the common-ion salt), with bromide having the larger effect compared to chloride. For penetrating counterions, such as salicylate, 26dichlorobenzoate, and tosylate, the micellar size and flexibility highly depend on the structure of the counterions. Mixed counterions (penetrating and non penetrating) have also been studied, and the micellar size found to increase dramatically in mixed counterions systems.